1. Field of the Invention
The present invention relates to piezoelectric thin film devices used in piezoelectric devices, and particularly to piezoelectric thin film devices used in high frequency bands, and a manufacturing method for the same.
2. Description of the Related Art
Recently, high frequency bands in the GHz range are being used in wireless communication systems such as mobile communication devices, and local area network (LAN) systems transferring data between computers at high transfer rates. Piezoelectric thin film devices are receiving attention in applications such as high frequency (RF) devices of high frequency electronics of such wireless communications systems. Applications as devices such as resonators, variable capacitors, and micro switches, for example, are expected uses for piezoelectric thin film devices. Piezoelectric thin film devices according to micro electro mechanical system (MEMS) technology are manufactured using thin film micro fabrication processes in the same manner as for semiconductor devices such as metal insulated semiconductor (MIS) integrated circuits. Therefore, it is possible to integrate MEMS piezoelectric thin film devices and semiconductor devices on a common semiconductor substrate.
Surface acoustic wave (SAW) devices, for example, are generally used as high frequency resonators. However, the resonant frequency of SAW devices exhibits an inversely proportional relationship to the gap distance between comb type electrodes. Since, at a frequency range exceeding 1 GHz, the gap distance between comb type electrodes of SAW devices is 1 m or less, it is difficult to correspond to recent higher usage frequencies demands.
In place of SAW devices, a film bulk acoustic wave resonator (FBAR), which makes use of a mode of longitudinal vibration in the thickness direction of a piezoelectric film, has recently been receiving attention as a resonator. The FBAR, which uses a piezoelectric film, is also occasionally referred to as a bulk acoustic wave (BAW) device. In the FBAR, resonant frequency is regulated by the acoustic velocity and film thickness of the piezoelectric device. For example, the piezoelectric film corresponds to an average film thickness of about 1 μm to about 2 μm at a band of 2 GHz, or a film thickness of about 0.4 μm to about 0.8 μm at a band of 5 GHz. By thinning the piezoelectric film further, it is possible to increase frequency by several tens of GHz.
In the structure of a current representative FBAR, a piezoelectric film of a material such as aluminum nitride (AlN) or zinc oxide (ZnO) is sandwiched between two opposing electrodes, more specifically, a top electrode and a bottom electrode. For improved performance, a resonator of the FBAR type is disposed so as to be suspended above a cavity. A fabrication method for an FBAR having a cavity is disclosed (refer to Japanese published unexamined application No. 2000-69594). For example, a hollow portion can be formed by anisotropic etching on a silicon (Si) substrate. Next, a sacrificial layer of an easily etched material, such as boron and phosphorous doped silicon glass (BPSG), for example, is filled into the hollow portion and planarized. A bottom electrode, a piezoelectric film, and a top electrode, respectively, are then stacked in sequence on top of the planarized sacrificial layer. Afterward, a hole is bored from the top electrode, which is formed above the sacrificial layer, to extend to the sacrificial layer. The sacrificial layer is removed by selective etching, forming the cavity.
Piezoelectric characteristics of a piezoelectric film used in FBARs are dependant upon orientation. In AlN piezoelectric films, for example, there is a strong mutual relationship between a full width at half maximum (FWHM) of c-axis orientation of an AlN crystal and an electromechanical coupling constant (refer to Rajan S. Naik et al., “Measurements of Bulk, C-Axis Electromechanical Coupling Constant as a Function on AlN Film Quality”, IEEE transactions on ultrasonics, ferroelectrics, and frequency control, Vol. 47, No. 1, pp. 292-296, January 2000). In order to attain desired piezoelectric qualities, it is essential to form a piezoelectric film so that the c-axis direction is oriented along the direction in which the bottom electrode and the top electrode oppose each other. However, there are limits on the orientation of the AlN piezoelectric film formed on top of the sacrificial layer, which raises the problem of a small electromechanical coupling constant.
In order to improve the orientation of piezoelectric crystal, there is a manufacturing method of an FBAR by epitaxially growing a piezoelectric film on a substrate (refer to Japanese published unexamined patent application No. 2001-94373). In the method disclosed in Japanese published unexamined patent application No. 2001-94373, an AlN piezoelectric film is epitaxially grown in (0001) orientation, more specifically, in the c-axis direction, on an Si substrate of (111) orientation. Atop electrode is formed on top of the AlN piezoelectric film. Afterward, the Si substrate is removed by anisotropic etching from the underside of the substrate until the AlN piezoelectric film is exposed, forming a via hole. After the AlN piezoelectric film has been exposed, the bottom electrode is formed from the underside of the substrate. Thus, a resonator using the epitaxial AlN piezoelectric film is formed above the cavity.
In the above mentioned manufacturing method, in order to orient the AlN piezoelectric film along the c-axis, it is essential to use a Si substrate in the (111) orientation. It may be a problem to use the (111) orientated substrate, which is different from the (100) orientated substrate used in general manufacturing methods of semiconductor devices.
Additionally, piezoelectric thin film devices such as variable capacitors or micro switches, have a movable electrode provided on an actuator, and a fixed electrode provided on a surface of a substrate which opposes the actuator. The actuator is supported on one end so as to be suspended over a substrate. The actuator changes the distance between the movable electrode and the fixed electrode. Piezoelectric actuators using an electrostriction effect or a reverse piezoelectric effect of a piezoelectric film, as a driving force, are currently being tested.
Lead zirconate-titanate (PZT) is a material known as a piezoelectric film having a large electrostriction effect. On PZT, in order to attain a film of good quality, it is essential to execute annealing at a temperature of about 600° C. after forming the film at room temperature. Because volume contraction occurs due to the annealing, the residual distortion of the PZT film will inevitably increase. The piezoelectric actuator is suspended in the air, and has a long and thin beam structure, which contains a piezoelectric layer sandwiched by the top and bottom electrodes. Therefore, it is difficult to suppress warpage occurring on the PZT piezoelectric film by residual distortion. Because a piezoelectric film of materials such as AlN, or ZnO can be deposited at near room temperature, it is possible to precisely control residual stress by film deposition conditions, compared to the PZT piezoelectric film. However, the electrostriction effect of materials such as AlN, or ZnO is small compared with PZT. Therefore, it is possible to have a small electromechanical coupling constant of the piezoelectric film, which may not be sufficient to ensure a sufficient drive range of the piezoelectric actuator.